1. Field of the Invention
The present invention relates to a tomography imaging apparatus used when a tomographic image of a subject is provided in medical or industrial field by applying a method by Optical Coherence Tomography (hereinafter, referred to as OCT) constituted by combining a light source for outputting light having a short coherence length and an equal light pass length interferometer of a Michelson interferometer or the like, particularly relates to a tomography imaging apparatus which does not need a reference face scanning mechanism for providing image data in a depth direction of a subject.
2. Description of the Related Art
In recent years, in a field of taking an image of a subject for medical use or industrial use, particularly in a field of an electronic endoscope, there is known an apparatus of taking a tomographic image of a subject by using a method of OCT.
Since light is used as a detection probe, the tomography imaging apparatus by OCT does not pose a problem of exposing a subject to X-ray irradiation as in an X-ray image taking apparatus of a background art, and is extremely preferable particularly when the subject is the human body. Further, a large-sized apparatus such as CT, MRI or the like is not needed, a subject can simply be inspected and therefore, a burden in view of cost and a burden in view of physical strength of a subject can be alleviated, which is preferable also in this respect.
Further, since interfered wave information at respective positions in a depth direction of a subject is provided by utilizing low coherence performance of light having a wide band of a spectrum width, the tomography imaging apparatus using OCT can detect reflected light from inside of a subject by a spatial resolution of μm order and can considerably promote a measurement resolution in comparison with an X-ray imaging apparatus of a background art.
A tomography imaging apparatus using OCT having such a number of excellent properties is disclosed in, for example, Optics vol. 32-4 (2003): by Manabu Sato, Naohiro Tanno or the like shown below.
FIG. 5 shows an outline of a tomography imaging apparatus of a background art. That is, an output from a low interferable light source 310 is made to be incident on an optical fiber 321. A light flux progressing in the optical fiber 321 is separated into two light fluxes by a 2×2 coupler 325, one of the fluxes is guided to a side of a subject 331 by an optical fiber 322, and other thereof is guided to a side of a reference mirror 342 by an optical fiber 323.
An object converging lens 332 is provided at a post stage of a light emitting end of the optical fiber 322 and the light flux is converged to the subject 331 by the lens 332.
On the other hand, light emitted from a light emitting end of the optical fiber 323 is irradiated to the reference mirror 342 via a collimator lens 341, the reference mirror 342 is made to be movable in an optical axis direction, and the reference mirror 342 is moved to a position at which an optical path length from the light emitting end of the optical fiber 322 to an observing position in a depth direction of the subject 331 and an optical path length from the light emitting end of the optical fiber 323 to the reference mirror 342 are equal to each other. Thereby, there is constructed an interferometer of a so-to-speak Michelson type capable of interfering light even by low interferable light, and interfered wave information of respective positions in the depth direction of the subject 331 is provided.
Reflected light from the observing position of the subject 331 and reflected light from the reference mirror 342 respectively regress on irradiation paths thereof, combined by the 2×2 coupler 325 to interfere with each other, the interfered light reaches an optical detector 352 via an optical fiber 324 and the interfered wave information is detected by the optical detector 352. Thereafter, the interfered light information detected by the optical detector 352 is converted into an electric signal and is inputted to a computer 365 via an amplifier 362, a band pass filter 363, and an A/D converter 364 to subject to predetermined image processing.
Such an OCT technology is referred to as TDOCT (Time Domain OCT) in which a drive mechanism of repeatedly moving the reference mirror in the optical axis direction is needed and the apparatus is large-sized and complicated.
Hence, there has been developed a technology referred to also as SDOCT (Spectral Domain OCT) dispensing with a drive mechanism for moving a reference mirror. The SDOCT is arranged with, for example, a diffraction grating and a Fourier transformation optical system on a face at which the reflected light (signal light) from the subject and the reflected light (reference light) from the reference mirror are superposed, and by subjecting the provided interference wave information to Fourier transformation operation, a tomographic image of the subject can be provided without driving the reference mirror.
Such an SDOCT technology described in JP-A-2001-272332, shown below, is known.
Further, the technology described in JP-A-2001-272332 further adopts an angular dispersion imaging method capable of directly detecting an envelope of an interference signal.
However, according to the above-described technology described in JP-A-2001-272332, an interference signal representing an optical intensity distribution of interfered light provided by the optical detector is multiplied by a cosine function constituting a variable by a phase difference Δ1 of signal light and the reference light as a weight and therefore, an amplitude cannot accurately be detected when an angle θ of the function becomes π/2, 3π/2 or the like.
Further, the optical members starting from the diffraction grating are arranged between light emitting faces and light detecting faces of the signal light and the reference light and an apparatus having a simpler constitution has been desired.